Laziness might not be the only reason why some people have so much trouble staying physically fit. Removing just two genes from mice completely saps them of their will to exercise, and a similar process might exist in humans.

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Researchers at McMaster University performed an experiment on a group of healthy, specially-bred mice. While the control group was unchanged, the researchers removed two genes from the muscles of the experimental group. These genes control the protein AMP-activated protein kinase, or AMPK, which is an enzyme essential for proper body functioning during exercise.

The mice then had to run for as long as they could - usually, that's a pretty easy task, since mice are excellent runners. But that's not what happened here, as researcher Gregory Steinberg explains:

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"While the normal mice could run for miles, those without the genes in their muscle could only run the same distance as down the hall and back. It was remarkable. The mice looked identical to their brothers or sisters but within seconds we knew which ones had the genes and which one didn't."

What causes the difference? The key appears to be the amount of mitochondria. Those mice without the AMPK genes had significantly lower levels of mitochondria, meaning they had trouble keeping up energy levels and taking in glucose. This is the first demonstrated link between AMPK and exercise, and Steinberg believes that at least some of the people who have trouble exercising might actually be having difficulty with their AMPK genes. He adds:

"As we remove activity from our lives due to emerging technology, the base level of fitness in the population is going down and that is reducing the mitochondria in people's muscles. This in turn makes it so much harder for people to start exercising."

All that said, while problems with the AMPK genes could have some serious impact on people's ability to keep fit, it's not clear what percentage of the population are actually affected by these issues. This study definitely is no reason to stop trying to stay healthy. At least, that's what I plan on keep telling myself as I force myself to go back to the damn gym.